The present invention relates to a high-efficiency driver circuit for capacitive loads, which circuit comprises a drive portion connected to at least one end of a capacitive electric load being applied a voltage waveform. The invention relates, particularly but not exclusively, to a driver circuit for a piezoelectric type of a printer head, this description making reference to this field of application for convenience of illustration only.
As is well known in this technical field, the electric characteristic of a comprehensive class of transducers, such as those described in U.S. Pat. No. 5,895,998, can be likened to a capacitive load. The piezoelectric heads of printers may also be viewed as capacitive loads. These printer heads are driven by applying a voltage waveform across their terminals. The waveform typically consists of a succession of linear edges. An example of this is to be found in U.S. Pat. No. 4,767,959 granted to Nippondenso Co.
The high degree of accuracy necessary in applying said voltage waveform to the capacitive load, along with the high frequency of the drive signal, has led to the use of linear-mode driver circuits, involving high power dissipation even though the load itself may be a non-dissipative one. FIG. 1 is an example of prior art wherein a linear drive is used for applying a voltage waveform to a capacitive element.
It should be emphasized that the capacitive load is a non-dissipative type, and that all the electric power is dissipated through the transistors that comprise the linear drive stage. Thus, the efficiency of this conventional approach is poor because of the large amount of power that is dissipated through the transistors. The technical problem that underlies this invention is to provide a driver circuit, particularly for capacitive loads, with appropriate structural and functional features for the load to be driven with high efficiency and without distorting the quality of the waveform generated across the capacitive load.
An embodiment of the present invention relates to a high-efficiency driver circuit for capacitive loads, whereby a voltage waveform formed by a succession of linear edges can be applied to a capacitive load using a combined switching and linear drive technique. The embodiment comprises: (1) an electronic circuit for applying a voltage waveform to a capacitive load, (2) a switching circuit connected to the capacitive load for supplying a fraction of current demanded by the capacitive load, and (3) a control circuit for controlling the switching circuit. Typically, the switching circuit comprises a transistor bridge powered between a first voltage reference and a second voltage reference having an inductor between the bridge nodes.
Features and advantages of a circuit according to the invention are made apparent by the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.